1. Field of the Invention
This invention relates in general to a funnel for cathode ray tube (CRT) and a mold for making same. More particularly, the invention relates to a technology for improving a positioning reference that is formed on an outer wall of a body of a funnel for cathode ray tube.
2. Description of Related Art
As well known in the conventional art, the glass bulb for cathode ray tube comprises a panel that forms a front portion and images are displayed thereon; a funnel that is located behind the panel, sealed to the panel, and a deflection yoke is externally mounted thereon; and a neck that is fusion bonded with the funnel and an electron gun is internally installed.
The panel comprises a face portion with an effective screen for displaying an image, and a skirt portion having a substantially rectangular enlarged opening that is connected to the face portion through a blend R portion and is used to seal to the funnel. The skirt portion comprises side portions each of which is between two adjacent corners among four corners.
As shown in FIG. 5, the funnel 1 comprises a substantially rectangular enlarged opening (seal edge surface) 2 to be sealed to the panel, and a substantially circular small-diameter opening 3 to be fusion bonded with the neck. The body portion 4 of the funnel 1 using the tube axis X as a central axis line has body structures 4a, and each body structure 4a that exists between two adjacent corners 5 among four corners 5 at the side of the enlarged opening 2 is gradually shrunk towards the side of the small-diameter opening 3. In addition, the neck is substantially a cylindrical shape.
As a cathode ray tube, in order to obtain a structure for displaying a proper image without color deviation, etc., it is important that the central axis lines of the panel, the funnel and the neck are accurately positioned one another with respect to the tube axis of the glass bulb and then assembled together. Therefore, a plurality of positioning references 6 (three entities, for example), which is in contact with fixtures to determine a position relative to the panel when the funnel is sealed to the panel, is disposed on the adjacent body structures 4a of the funnel 1. The positioning reference 6 is made in a convex shape whose top portion has a reference surface 6a that contacts with the fixture. The reference surface 6a is formed in such a manner that a predetermined distance is set from the central axis line of the funnel.
In addition, as shown in FIG. 6, when manufacturing the funnel 1, after a high-temperature molten glass gob is provided to a female mold comprising a bottom mold 7 and a shell mold 8, a plunger 9 as a male mold is moved downwards to press and extend the glass gob. After the molten glass gob is formed into predetermined shape, the plunger 9 is moved upwards. Furthermore, after the shell mold 8 is removed, the funnel 1 is removed out of the bottom mold 7.
In this situation, a molding portion corresponding to the positioning reference 6 is formed as a recess portion on the bottom mold 7. In addition, considering the easiness of taking the funnel 1 out of the bottom mold 7 after being molded, the molding portion corresponding to the positioning reference 6 is formed on the uppermost portion of the bottom mold 7 in a status that the upper portion is open. Therefore, the positioning reference 6 is located in the vicinity of a mold match line M formed by the mold matching of the bottom mold 7 and the shell mold 8.
In addition, it is a common example that the reference surface 6a of the positioning reference 6 is formed in substantially parallel with the central axis line X of the funnel 1. The shapes of the positioning reference 6 have types as shown in FIGS. 7A and 7B, for example. As shown in FIG. 8, for any above type of the positioning reference 6, the edge 6b (front edge, hereinafter) at the side of the seal edge surface 2 of the reference surface 6a is formed by projecting from an outer side of the mold match line M (a side away from the central axis line X, following is the same). For example, the projection dimension L from the mold match line M to the front edge 6b of the reference surface 6a can be set about 1.5 mm˜2.5 mm.
The aforementioned projection dimension L is previously set by predicting a dimension unevenness generated when the funnel 1 is pressed to form, so that currently, the value of the projection dimension L is determined according to operator's experience. Furthermore, as shown in FIG. 9, when molding the positioning reference 6, a sliding plate 21, at a front end of which a molding surface 21a for molding the reference surface 6a of the positioning reference 6 is provided, is fitted slidably in the x-y direction into a slit 20 formed at the uppermost part of the bottom mold 7. The position of the sliding plate 21 is suitably adjusted to secure a proper dimension accuracy of the reference surface 6a. 
Conventionally, the reference surface 6a of the positioning reference 6 is projected more outwards than the mold match line M by about 1.5 mm˜2.5 mm, causing following problems.
Since the glass thickness of the forming portion of the positioning reference 6 on the funnel 1 is overly thick in comparison with the other portions, when press molding of the molten glass gob, the temperature balance in the vicinity of the positioning reference 6 is degraded, there might be possibilities of dimensional defects and deformation. In addition, since the positioning reference 6 becomes a cause to interfere the flow of the molten glass, the drawback such as sag or tip on the glass is caused.
In addition, since the positioning reference 6 is formed by the molten glass at the final stage of the pressing and extending by the plunger 9, wrinkles, etc. might be possibly created thereon because the fluidity of the molten glass is decreased.
Furthermore, in a slidable mold such as using the aforementioned sliding plate 21, when the sliding plate 21 is made to move in the X direction, an acute angle gap is formed between the front end of the sliding plate 21 and the recess 22 of the bottom mold 7. In addition, even when the sliding plate 21 is made to move in the Y direction, a tiny gap also exists between the sliding plate 21 and the slit 20. As a result, tiny angular protrusions or tiny line-shaped protrusions are formed on the border of the reference surface 6a. These do not only become the causes of creating the crack, etc., but the quality deterioration or the degradation of molding yield is caused.